Four billion miles away and fading fast, NASA's Pioneer 11 has one last mission to accomplish before drifting into oblivion.
This week or next, the National Aeronautics and Space Administration will test the power of Pioneer 11, the fourth-most distant spacecraft. Project directors want to see how well the probe works with reduced voltage before it loses all power.
The results of these tests could extend the life of Pioneer 11's healthier sister ship, Pioneer 10, the most distant human-made object at 5.8 billion miles away.
Pioneer 11, which was launched in 1973 and became the first spacecraft to explore Saturn six years later, is expected to have insufficient power to operate its science instruments after August.
Both Pioneers swept past Jupiter in the mid-1970s and provided a detailed look at the magnetosphere and radiation belts surrounding that planet. Pioneer 11 went on to Saturn and found a magnetosphere and radiation belts there, too, as well as an additional ring and moon.
To NASA's surprise, the probes kept measuring cosmic radiation and sending back data as they plowed deeper into space.